1. Field of the Invention
The present invention relates generally to optical fiber interconnects. More specifically, the preferred embodiment of the present invention is related to heat molding a plastic optical fiber to a light emitting diode.
2. Description of the Prior Art
Two types of optical fibers are currently in general use. First, "large diameter" plastic optical fibers having a diameter of approximately 14 mills with an additional cladding of approximately one mill are used for short transmission path lengths. "Small diameter" optical fibers having a silica or a glass inner core of approximately 8 mil diameter have a relatively thick cladding giving an overall diameter of approximately 26 mils. This second type of optical fiber has an attenuation characteristic substantially smaller than that of plastic optical fibers, thus is more suitable for long transmission path lengths. However, due to the small diameter of the optically transmissive core of silica and glass optical fibers, it is much more difficult to efficiently couple and launch sufficient power from a light source into the silica or glass optical fibers. This difficulty in coupling has been a serious disadvantage in the use of small diameter optical fibers in commercial applications.
Laser diodes can be constructed to confine the active light emitting region to a narrow 2-3 mm wide stripe that can be axially coupled into small optical fibers with high efficiency. However, this has not been the case for LED's which characteristically have larger light emitting surfaces and often have light emitting side surfaces. LED's have a long operational lifetime, are simple to use, and are less temperature sensitive than laser diodes. Accordingly, it is desirable to efficiently couple LED's to small diameter optical fibers.
It has proven to be no easy task to align two fibers, or a light source and a fiber, with enough precision to allow the light from one to enter the other efficiently. Most prior art interconnects are chiefly concerned with a close, end-to-end mating and precise alignment. These interconnects can be divided into two types, those that mount optical fibers to the light source with epoxy--the "wet" interconnects--and those using aligning rods, pins, or grooves without epoxy, --the "dry" interconnects.
Epoxy-based wet interconnects make good use of low-cost materials such as plastic-clad silica fibers and plastic connector housings. However, the manufacture of these interconnects is usually labor-intensive, time consuming and expensive. Special fixtures are often required to align the interconnect and a technician must wait for the epoxy to set thoroughly before the fiber can be processed further.
The most popular alignment mechanism for dry interconnects consists of a series of small rods or pins inside a metal sheath. The optical fiber is inserted and held in a narrow space between the rods. A good review of both wet and dry fiber optic interconnects can be found in "FIBER OPTICS GROWING STRONG," Ohr and Adlerstein, Electronic Design 23, Nov. 8, 1979.
The prior art in coupling plastic or glass core optical fibers to light sources such as light emitting diodes (LED's) typically involves mechanical alignment and clamping of the optical fiber to the light emitting diode. FIG. 1 illustrates three methods used in the prior art to couple optical fibers to light emitting diodes. In FIG. 1A an optical fiber 10 has been epoxied in alignment with a light emitting diode LED 20. In FIG. 1B a silica core optical fiber 30 is coupled to a LED 20 in combination with a spherical lens 40 to converge light from LED 20 to the small silica core 50 of optical fiber 30 and epoxy to hold spherical lens 40, and optical fiber 30 in alignment. FIG. 1C illustrates a dry interconnect having an optical fiber 10 mechanically aligned to an LED 20 by a crimp-on connector 55. Note that the end of optical fiber 10 is held apart from LED 20 to prevent deterioration of the optical surfaces resulting from their rubbing together and scratching.
There are several disadvantages to these prior art methods. First, there are undesirably large light losses associated with the optical coupling between the LED and the optical fiber. This light loss is caused by a number of factors including misalignment between the optical fiber and the light emitting diode, the necessary spacing between the optical fiber and the LED and difficulties in converging the light from the LED into the receptive cone of the optical fiber, especially where the optical fiber is a small diameter silica or glass. A practical difficulty related to coupling optical fibers to LED's is the time and expense required in the alignment and coupling process. These disadvantages and difficulties in current methods for coupling optical fibers to LED's have been limiting factors in the use of optical fibers in electrical-optical circuits.
A major cause of these difficulties arises from the fact that it is difficult to abut the optical fiber to the LED without damaging the wire bond attached to the surface of the LED or causing a deterioration in the optical surfaces. The optical fiber must be suspended away from, but held in alignment with the LED during the coupling process. This gap between the LED and the end of the optical fiber is not only a major cause of attenuation in itself, it also gives rise to misalignment and difficulties in mechanical coupling.